Tuesday, June 22. 2010

I know a lot has been said about this beautiful value we affectionately call NULL, which is neither here nor there and that manages to catch many of us
off guard with its casual neither here nor thereness. Database analysts who are really just back seat mathematicians in disguise like to philosophize
about the unknown and pat themselves on the back when they feel they have mastered the unknown better than any one else. Of course database spatial analysts, the worst kind of back seat mathematicians,
like to talk not only about NULL but about EMPTY and compare notes with their brethren and write dissertations
about what to do about something that is neither here nor there
but is more known than the unknown, but not quite as known as the empty string.

Okay getting to the point, one of our clients asked us about a peculiar problem they had with a query, and the strange results they were getting. We admit this still manages to catch us off guard every once in a while.

Many will be surprised to find out that Case 1 yields no records and wonder how its different from case 2. Some may also wonder why case 5 does not return the records with null values. This is not a bug in PostgreSQL, but just the nature of NULL that you will see in other ANSI SQL compliant non-PostgreSQL databases as well.

The other interesting thing demonstrated by the example above, is the idea of a unique index vs. a primary key. In a primary key, you can't have nulls, but in a unique key, you can have as many nulls as you want, but other values must be unique. This is again ANSI standard compliant behavior
you will see in other databases.

Why does case 1 return no records?

Since the NOT IN subquery has NULLS, which is another way of saying UNKNOWN, you can't be aboslutely sure that a value is not in the unknown set, but if asked to say true or false what do you say in such a case? The answer according to the ANSI standard is false.

Why does case 5 not return NULL records?

You can't say a missing value is equal to a set of missing or non-missing values, but should you return true or false when asked to do so? The answer according to the ANSI standard is false.

Just say NO

So in short, when in doubt about any question that requires a Yes or NO answer, just say NO.

THe STRICTNESS problem

This as odd as it may seem is also related to the STRICTNESS issue we brought up that broke the inlining logic we had in place for PostGIS functions. As Anarazel beautifully explains in our bug ticket
that all boolean expressions are fundamentally considered to be not strict meaning they expect a true or false answer (not unknown). What STRICTNESS means is that when any of the inputs to your function is NULL, your answer should be NULL, but in the case of booleans, things often get messy when you ask
for data and the computer answers, "I DON'T KNOW, IF I SHOULD GIVE YOU THIS DATA".